The present invention relates to a method and system of obtaining a spatially representative sample of fluid flowing in a duct, and particularly relates to a method and system of obtaining a spatially representative sample of fluid utilizing multiple sampling probes.
Emissions flowing in an exhaust stack of a gas turbine have been routinely sampled for many years. This sampling may indicate when emissions contain certain concentrations of pollutants. It is thus necessary to ensure that the emissions are sampled accurately.
Non-representative gas sampling in exhaust streams is a significant contributor to inaccurate low level gas species (emissions) measurements. For gas turbine applications, many units are being certified at 9 ppm and even as low as 2 to 3 ppm, all normalized to a diluent level of 15% oxygen. In these applications, the effect of a non-representative oxygen sample being off by 0.5% translates to a 7%–15% bias in NOx emissions depending on the excess oxygen exhaust concentration. Additionally, NOx and other pollutant species like CO and NH3 can be highly stratified resulting in large variations of species concentrations.
A current process to achieve representative sampling of an exhaust stream involves measuring multiple points across the stack. This current process is generally a manual process that is not suitable for continuous monitoring systems. Points sampled depend on test method: for 40 CFR 60, App A, Method 20 (turbines) eight points at the lowest O2 levels are sampled, for RATAs (40 CFR 60, App. B) three points are sampled, and for Part 75 between one and three points are sampled.
While multipoint sampling is widely used to obtain a representative sample of fluid, it typically involves a manual process of inserting a sample probe to various locations in the fluid stream. This sampling process is thus laborious and time consuming as it requires multiple measurements to determine the gas velocity and then full-time attendance of a sample metering pump to draw flow through a port of known area at specific flow rates.
When manually sampling at the various locations in the stream, the gas is extracted at equal gas volumes per point. This approach ensures a volume averaged gas concentration across the flow. Obtaining volume averaged flow requires point information on the gas volume flow rate and temperature and point specific flow rate control.
Simple solutions to achieve multipoint sampling often involve using a single probe with multiple sampling holes spaced along the probe length. However, because it involves a common sample line, this approach does not allow easy and on-line adjustment of the flow rate sampled at each point.
A variation to this solution employs a sample probe with critical pressure drop at the sample probe inlets. A sample pump draws flow into the multiple probe inlets at equal volumes independent of the probe location. This avoids problems with variation in flow due to pressure drop along the sampling probe, so sampling points further into the flow are equally represented. However, it does not provide an isokinetic flow. For example, when sampling in a low flow and high flow region, both points are equally represented. This sampling biases the true impact of the low flow. If the low flow region contained twice as much pollutant concentrations but only half as much flow as the high flow region, then the overall emissions would be overly biased (i.e., biased high). As a quantitative example of this overly biased sampling, suppose a low flow region constituted 25% of the entire exhaust flow and contained 10 ppm of NOx and a high flow region constituted 75% of the entire exhaust flow and contained 5 ppm of NOx. In this quantitative example, the flow averaged emission is OLE—LINK1(25%)(10 ppm NOx)+(75%)(5 ppm NOx) OLE—LINK1=6.25 ppm NOx. However, the sampling system would determine the result as 7.5 ppm NOx via the following calculation: (50%)(10 ppm NOx)+(50%)(5 ppm NOx)=7.5 ppm NOx.
In sampling systems where critical pressure drop is not established at the port inlet, further bias can be introduced due to sample line length and pressure head differences. In these cases, sampling further into a flow stream would have higher line pressure losses and lower sampling rates. Assuming the above quantitative example, if the high flow region was in the center of a stack (i.e., center of the flow) and the low flow region was closer to the wall of the stack, and the high flow region formed 45% of the total flow and the low flow region formed 55% of the total flow due to sample line pressure differences, the determined results would be further biased at 7.75 ppm NOx as calculated as follows: (55%)(10 ppm NOx)+(45%)(5 ppm NOx)=7.75 ppm NOx.
Other systems utilize a sampling grid having multiple sampling probes spatially distributed across the flow field. In these systems, the flow is typically drawn through a common pump and is sequenced to get point-to-point sample concentrations rather than average sample concentrations.
There thus remains a need for a method and system of obtaining a more spatially representative sample utilizing multiple sampling probes.